Capacitive material presence detecting apparatus

ABSTRACT

Apparatus for detecting the amount or presence of material in a detection zone comprising a first electrode, input signal means for supplying an electric input signal to the first electrode; a second, smaller electrode parallel to the first electrode; the detection zone being located between the first and second electrodes; a third electrode which is larger than the second electrode and substantially surrounds the second electrode in a plane parallel to the first electrode, and receiving means for detecting signals on the second electrode, the signal on the second electrode being responsive to material in the zone. The second and third electrodes are not electrically connected but are maintained at near the same electric potential whereby the electric field between the first and second electrodes is, in use, substantially linear and fringing fields, if any, occur only between first and third electrodes at their outer edges.

This invention relates to detecting apparatus.

More particularly, the invention relates to detecting apparatus fordetecting the amount or presence of material in a detection zone.

In many manufacturing systems, it is of importance to know veryaccurately the number of articles which have been placed in a package.In some circumstances it is not possible to obtain satisfactory checkinginformation by visual inspection or by weighing. It is thus an object ofthe invention to provide detecting apparatus which can provide veryaccurate information as to the amount or presence of material in adetection zone. The packages are conveyed into the detection zone andthe amount or quantity of material in the packages can be monitored bythe apparatus of the invention.

According to the present invention there is provided apparatus fordetecting the amount or presence of material in a detection zone, saidapparatus comprising:

a first electrode;

input signal means for supplying an electric input signal to the firstelectrode;

a second electrode, said detection zone being located between said firstand second electrodes;

a third electrode which substantially surrounds the second electrode andis larger than the second electrode; and

receiving means for detecting signals on the second electrode, thesignal on the second electrode being responsive to material in the zone.

Preferably, the second and third electrodes are not electricallyconnected but are maintained at near the same electric potential wherebythe electric field between the first and second electrodes is, in use,substantially linear and fringing fields, if any, occur only between thefirst and third electrodes at their outer edges.

Preferably the second electrode is narrow so that a narrow slice ofmaterial in the zone is detected. This enables very accurate monitoringso carried out compared to known techniques which, generally speaking,scan only relative wide areas and are affected by fringing fields.

Apparatus of the invention can be very conveniently arranged to operateon a conveyor line upon which packaged articles pass. The articles passthrough the detection zone and hence the articles pass between the firstelectrode and the second and third electrodes. Preferably the firstelectrode comprises a plate over which the article pass and the secondand third electrodes are located above the first electrode. The inputsignal means may comprise an oscillator for applying a sinusoidalvoltage to the first electrode. The detecting means may comprise anoperational amplifier having a known capacitive feedback loop. Thearrangement is such that articles in the detecting zone, especiallynon-metallic articles, alter the effective dielectric properties withinthe zone thereby altering the effective capacitance between the firstand second electrodes. This alteration is reflected at the output of theoperational amplifier and with suitable detecting circuitry can be usedto provide information as to the number of articles in a package or thequantity of material in a package.

In accordance with the present invention there is also provided adetection circuit for detecting the amount or presence of material in adetection zone, said circuit comprising:

a first electrode and a second electrode, said detection zone beinglocated between said first electrode and said second electrode, and saidfirst electrode being supplied, in use, with an electric input signal;and

an operational amplifier with an input connected to said secondelectrode and having a capacitor in a feedback loop between said inputthe output of the amplifier, the signal outputted by the amplifier beingresponsive to the material in the zone.

In accordance with the present invention there is also provided adetection device for use in detecting the amount or presence of materialin a detection zone, said device comprising:

a first electrode;

a second electrode substantially parallel with respect to and oppositesaid first electrode; and

a third electrode which substantially surrounds, is substantiallyelectrically isolated with respect to and is larger than said secondelectrode;

said detection zone being located between said first electrode and saidsecond electrode.

The invention will now be further described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view of detection apparatus of the invention;

FIG. 2 shows one configuration of the electrodes;

FIG. 3 shows another configuration of the electrodes;

FIG. 4 is a block diagram of a preferred embodiment of a detectioncircuit of the detection apparatus;

FIG. 5 is a circuit diagram of a received signal amplifier and theelectrodes of the detection circuit;

FIG. 6 is a diagram of a phase detection circuit of the detectioncircuit;

FIG. 7 is a first phasor diagram illustrating generation of a detectionsignal of the detection circuit;

FIG. 8 is a second phasor diagram illustrating generation of a detectionsignal of the detection circuit; and

FIG. 9 is a diagram of a detection signal generated by the detectioncircuit.

The detection apparatus diagrammatically illustrated in FIG. 1 comprisesa first electrode 2 which comprises a metallic plate. Spaced from theelectrode 2 are second and third electrodes 4 and 6. As best seen inFIG. 2, the third electrode 6 is parallel to the first electrode 2 andis the same size as the first electrode. The second electrode 4 is inthe form of a strip which is located in an opening 8 formed in the thirdelectrode 6. In use of the apparatus, articles to be monitored pass intoa detection zone 10 (shown in broken lines) which is between theelectrodes 2 and 4. A conveyor (not shown) conveys articles to the zone10 generally in the direction of arrow 12 so as to pass through the zone10. The presence of the articles affects the dielectric properties(especially the permativity) of the zone 10 and this can be monitored asexplained hereinafter.

The detecting apparatus includes an oscillator 14 the output of which iscoupled to the first electrode 2. The second electrode 4 is coupled toone input 15 of an operation amplifier 16. The other input 17 isearthened, as shown. A feedback capacitor 20 is coupled from the output22 of the amplifier 16 to the first input 15. The effective capacitanceof the first and second plates 2 and 4 is well defined and so the ratioof that capacitance with that of the capacitor 20 is also well defined.Thus any variation in the effective dielectric properties of the zone 10will affect the signal level present on the output 22 of the amplifier16.

When a package containing a correct number or amount of material passesthrough the zone 10, the output 22 will produce a "normal" response.When however a package having more or less material herein passesthrough the zone 10, the output signal will differ from the normalresponse. If it is above or below predetermined limits, appropriatesignals can be generated to indicate that the packaging has beenincorrect so that appropriate action can be taken with respect to thedefective packages.

The oscillator 14 is operable over a relatively wide range offrequencies. It is operable at least between 5 kHz to 1 MHz butpreferably is about 80 kHz. It has a sinusoidal input preferably 12volts peak to peak. The capacitance between the electrodes 2 and 4 istypically of the order of 0.1 pF and the capacitance 20 is typically 4.5pF. However the capacitance between the electrodes 2, and 4 can rangefrom 0.001 pF to several pF or more.

The amplifier 16 preferably is a high gain amplifier so that its input15 will be at virtual earth. This is an important feature because theelectrode 6 which is located very close to the electrode 4 is also atearth potential. Therefore the electric field from the plate 2 to theplate 4 passing through the zone 10 will be substantially linear becausethe field behaves as though the plates 4 and 6 form part of a singleplate because they are held at substantially the same potential. Anyfringing fields 24 will thus occur only between the peripheries of theplates 2 and 6, as indicated in FIG. 1. It will be appreciated that itis desirable to keep the gap between the second and third electrodes 4and 6 as narrow as possible. Typically the gap is less than 1 mm. Thegap can be filled if necessary with an insulating material. The spacebetween the plates 4 and 6 (and 2) can be varied in accordance with thesize of the packaging which passes between the electrodes. Thisvariation will require the apparatus to be recalibrated because of thechange of capacitance between the plates.

In some circumstances, it may be desirable to include more than oneplate corresponding to the second plate 4 so that a number of articlescan be scanned simultaneously on a conveyer line. Further, a pluralityof strip electrodes 26 may be provided in a side by side relationship,as shown in FIG. 3. Each of the strip electrodes functions analogouslyto the electrode 4 of FIG. 2. The strip electrodes 26 can be connectedto inputs of separate amplifiers or alternatively switching ormultiplexing techniques can be used to sequentially sample the outputsof the various electrodes 26.

In a further modification a two dimensional array of second electrodes 4may be provided in an opening in the third electrode 6.

It will be appreciated by those skilled in the art that many refinementscan be incorporated into the circuitry for processing of the signaloutput from the second electrode 4 and/or the amplifier 16 so as toeliminate the effects of noise, to give fast response times, etc.

A preferred detection circuit 50 of the detection apparatus isillustrated in FIG. 4. The detection circuit 50 includes a signalgenerator 52 which generates and supplies a sine wave voltage ofapproximately 80 kHz to a transmitted signal amplifier 54 and a phaseshift circuit 56. The transmitted signal amplifier 54 amplifies thesignal received from the generator 52 and supplies the amplified signalto the first electrode 2 of the apparatus. The signal produced on thesecond electrode 4 as a result of the signal applied to the firstelectrode 2 is inputted, as a received signal, to a received signalamplifier 58, which is also connected to the third electrode 6.

When an object passes between the first and second electrodes 2 and 4and the detection zone 10, the object, whilst altering the capacitancebetween the first and second electrodes 2 and 4, also alters theresistance between the electrodes 2 and 4. The equivalent electriccircuit 59 of the first and second electrodes 2 and 4 is illustrated inFIG. 5 and is shown coupled to the received signal amplifier stage 58.The equivalent circuit 59 comprises a variable capacitance and avariable resistance which are connected in parallel. The equivalentcircuit 59, of course, receives the signal outputted by the transmittedsignal amplifier 54 and outputs the received signal on the secondelectrode 4 to the input 60 of the received signal amplifier 58. Thereceived signal amplifier 58 includes two operational amplifiers 62 and64, the inverting input of the first amplifier 62 being the input 60 ofthe received signal amplifier 58. The non-inverting input of the firstamplifier 62 is connected to ground and to the third electrode 6, sothat the input 60 is held at virtual earth, as discussed with referenceto FIG. 1. The first amplifier 62 has a feedback loop to the invertinginput that includes a resistor and capacitor in parallel. Therelationship between the voltage at the output of the first amplifier 62and the voltage inputted to the first electrode 2 by the transmittedsignal amplifier 54 is given by equation 1 below. ##EQU1##

Where:

V_(o) is the voltage at the output of the first amplifier 62;

V_(i) is the voltage inputted by the transmitted signal amplifier 54 tothe first electrode 2;

ω is the frequency of the signal outputted by the transmitted signalamplifier 54;

R_(t) and C_(t) are the resistance and capacitance between the first andsecond electrodes 2 and 4; and

R_(f) and C_(f) are the resistance and capacitance in the feedback loopof the first amplifier 62.

The resistances R_(t) and R_(f) are relatively large and do not affectthe amplitude of the signal outputted by the first amplifier 62 to thesame degree as the effect caused by fluctuations in the capacitanceC_(t), due to the entry of objects into the detection zone 10. Asdiscussed previously with reference to FIG. 1, the fixed capacitanceC_(f) is about 4.5 pF and the capacitance between the electrodes 2 and 4varies over a wide range of pFs depending on the objects insertedtherein. Although the resistances R_(t) and R_(f) have negligable effecton the amplitude, it is desirable to consider them with respect to thephase change imparted on the inputted signal by the first and secondelectrodes 2 and 4 and the first amplifier 62. Some objects have littleeffect on the phase change whilst other objects have a marked effect andthis can be exploited in the detection process, as discussedhereinafter. The signal at the output of the first amplifier 62 ispassed, via a coupling capacitor 68 to the second amplifier 64, which isconfigured as an inverting linear amplifier. The output of the secondamplifier 64 is the output 70 of the received signal amplifier 58 andthe signal present at the output 70 will hereinafter be referred to asthe electrode signal.

The output 70 of the received signal amplifier 58 is connected to afirst input 72 of a phase detection circuit 74. A second input 76 of thephase detection circuit 74 receives a reference signal from the output78 of a reference signal amplifier 80, which amplifies the signaloutputted by the phase shift circuit 56. The phase shift circuit 56produces and outputs a square wave version of the signal generated bythe generator 52. The phase shift circuit 56 also introduces apredetermined phase shift with respect to the signal generated by thegenerator 52 so that the reference signal inputted to the phasedetection circuit 74 is a square wave of the same frequency as theelectrode signal but has a predetermined phase shift with respect to theelectrode signal.

The phase detection circuit 74, as shown in FIG. 6, comprises a phasesensitive detector 82 and a standard RC low pass filter 84, which isused to attenuate all of the AC components of the signal outputted fromthe phase sensitive detector 82. The phase sensitive detector 82receives the reference signal from the second input 76 and the electrodesignal from the first input 72. The phase sensitive detector 82superimposes the reference signal and the electrode signal and outputsthe result to the low pass filter 84. Thus, the reference signal samplesthe electrode signal periodically as desired. The signal produced at theoutput 86 of the low pass filter 84, which is the output of the phasedetection circuit 74 is a detection signal which is DC and has anamplitude which fluctuates in response to objects traversing thedetection zone 10. By virtue of these fluctuations in amplitude thedetection signal is effectively a low frequency signal. The remainder ofthe detection circuit 50 uses the detection signal as a basis fordetermining whether to accept or reject articles passing through thedetection zone 10 which correspond to the signal.

The voltage of the detection signal which appears at the output 86 ofthe phase detection circuit 74 is given by equation 2 set out below.##EQU2##

Where

V_(o) is a constant dc component;

V_(r) is the voltage of the reference signal;

V_(d) is the voltage of the detection signal;

V_(e) is the voltage of the electrode signal received at the input 72 ofthe phase detection circuit 74; and

φ is the difference in phase between the electrode signal and thereference signal received on the second input 76 of the phase detectioncircuit 74.

The phasor diagram shown in FIG. 7 illustrates the relationship betweenthe electrode signal and the detection signal produced when packetscontaining surgical gauze swabs are placed in the detection zone 10. Theswabs are extremely thin and do not significantly alter the phase of theelectrode signal on being inserted into the detection zone 10, as can beseen by comparing the voltage V_(e0) 90 of the electrode signal when thedetection zone 10 is empty with the voltage V_(e1) 92 when a packetcontaining one swab is present in the zone 10. By virtue of the welldefined capacitance C_(t) of the detection zone 10, however, thepresence of the swab causes a significant change in the capacitanceC_(t) which gives rise to a significant increase in the amplitude of theelectrode signal, V_(e1) 92, compared to the signal, V_(e0) 90, when thezone 10 is empty. This is also illustrated with respect to the voltagesproduced when two swabs are present in the zone 10, V_(e2) 94, threeswabs V_(e3) 96, and four swabs V_(e4) 98, as shown in FIG. 7. In thisinstance, the electrode signals V_(e0) 90, V_(e1) 92, V_(e2) 94, V_(e3)96 and V_(e4) 98 lag the reference signal by about 35° and give rise todetection signals V_(d0) 100, V_(d1) 102, V_(d2) 104, V_(d3) 106 andV_(d4) 108, respectively, as shown in FIG. 7. Thus the level of thedetection signal increases for each swab placed in the zone 10 and byanalysing the level of the detection signal produced a reject or acceptdecision with respect to a packet can be made. For example, if onlypackets containing three swabs are acceptable then only packages whichproduce a detection signal having a level which is approximately equalto that of V_(d3) 106 are accepted. An acceptance range or window isestablished which includes V_(d3) 106 and any packet which does notproduce a detection signal having a voltage within this window, when thepacket is centered on the detection zone, is rejected thereafter.

Whilst the apparatus of the present invention is particularlyadvantageous for detecting the presence of extremely thin objects, suchas surgical swabs, which do not produce significant changes in phase ofthe electrode signal, the circuit 50 may also be used to monitorobjects, such as food packs, which produce significant changes in thephase of the electrode signal. The phasor diagram of FIG. 8 illustratesone example where the phasor V_(e0) 110 is the voltage of the electrodesignal when the detection zone 10 is empty, V_(e1) 112 is the voltage ofthe electrode signal when the detection zone 10 contains a packageincluding one food pack, V_(e2) 114 two food packs, V_(e3) 116 threefood packs, and V_(e4) 118 four food packs. The phase of the electrodesignal changes significantly as further food packs are included in thedetection zone, with the change being approximately 15° to 20° for eachfood pack. As further food packs are inserted into the detection zone 10the capacitance C_(t) increases thereby causing an increase in themagnitude of the electrode signal V.sub. e0 110 to V_(e4) 118, as shownin FIG. 8. The significant phase shift introduced by the food packs isexploited to enhance the detection process by adjusting the phase of thereference signal, using the phase shift circuit 56, so that packages tobe accepted produce an electrode signal which is substantially 90° outof phase with respect to the reference signal. This gives rise to a zerolevel detection signal V_(d3) 126. Packages with more or less food packsthan the desired number produce detection signals which are eitherpositive, i.e. V_(d0) 120, V_(d1) 122, and V_(d2) 124, or negative, i.e.V_(d4) 126, and these packages are rejected. Thus the acceptance windowis set so as to accept packages which produce a substantially zero leveldetection signal, i.e. those containing three food packs.

The detection circuit 50 of the apparatus includes circuitry for makinga reject or accept decision on the basis of an analog signal andcircuitry for making a reject or accept decision on the basis of digitalsignals. The circuitry used may be selected as desired depending on thedetection operation to be performed. The decision based on the analogsignal is with respect to whether the detection signal exceeds athreshold level whereas the decision based on the digital signals iswith respect to whether the detection signal falls within an acceptancewindow.

The detection signal produced by the phase detection circuit 74 isinputted to a first low frequency amplifier 130 and a second lowfrequency amplifier 132. Both low frequency amplifiers 130 and 132 areused to amplify the detection signal within a low bandwidth and removeany high frequency interference signals, such as noise.

The output of the first low frequency amplifier 130 is passed to theinputs of a positive and negative rectifier 134, which rectifies anypositive part of the detection signal and places the result on a firstoutput 136 and rectifies any negative part of the detection signal andplaces the result in a second output 138. The two outputs 136 and 138are connected directly to the positive and negative inputs 140 and 142,respectively, of an adder and level detector circuit 144. The adder partof the adder and level detector circuit 144 inverts the polarity of anysignal appearing on the negative input 142 and adds the inverted signalto any signal appearing on the positive input 140. The sum signalproduced as a result of the addition performed by the adder part issubmitted to the level detector part of the circuit 144 and if the sumsignal exceeds a predetermined threshold then the output 146 of theadder and level detector circuit 144 goes low. If an analog controlswitch 148 is closed, the input 150 of a reject relay circuit 152 willreceive the signal on the output 146 of the adder and level detectioncircuit 144 and will also be pulled low, thereby activating a series ofreject relays (not shown).

The reject relay circuit 152 on receiving a low signal on the input 150first activates a relay or relays to cause indications, either audibleor visual, to be made to a user of the apparatus 50 that a packagepresent in the detection zone 10 is to be rejected. Secondly, after apredetermined delay to allow the package between first and secondelectrodes 2 and 4 to leave the detection zone 10, a further relay orrelays are activated by the reject relay circuit 152 to cause thepackage to be removed from the production line.

In some circumstances it may be desirable to submit the sum signalproduced by the adder part of the adder and level detector circuit 144to the level detector part of the circuit 144 at only certain periods oftime. For example, the sum signal which is of interest may be producedwell before the corresponding object is centrally disposed in thedetection zone 10. Thus the sum signal needs to be stored and thensubmitted after a delay to the level detector part so that any rejectsignal produced is not sent to the reject relay circuit 152 too early.To enable this to be performed a flow control circuit 154 is providedwhich receives and holds the sum signal until a trigger signal isreceived on an input 156. When the trigger signal is issued the flowcontrol circuit 154 inputs the stored sum signal to the level detectorpart of the adder and level detector circuit 144. The trigger signal isgenerated by a hold control circuit 158, as desired, in response tosignals received from the digital circuitry of the detection circuit 50.The signals inputted by the digital circuitry to the hold controlcircuit 158 are described hereinafter. The control of the sum signal fedto the level detector part of the adder and level detector circuit 144by the flow control circuit 154 and hold control circuit 158 may bedisabled or enabled as desired. When enabled, the level detector part isprevented from receiving the sum signal directly from the adder part.

The output of the second low frequency amplifier 132 is passed to areturn control circuit 160 and a comparator and latch circuit 162. Inthe comparator and latch circuit 132 six threshold levels are set asdesired, three relating to the analysis of negative detection signalsand three relating to the analysis of positive detection signals.Whether negative or positive detection signals will be analysed isdetermined by the manner in which the reference signal samples theelectrode signal, as discussed previously. The three threshold levelsfor each polarity are low Wl, high Wh, and over Wo and are used todefine, inter alia, the acceptance window.

For analysis of any detection signal three threshold levels are selectedone low Wl, one high Wh and one over Wo, each being positive or negativedepending on the intended fluctuation of the detection signal to beanalysed. The high level Wh is always selected to between the other twolevels Wl and Wo and the acceptance window to be between Wl and Wh,between Wh and Wo or anything over Wo. Anything lower than the low levelWl is to be ignored and thus the acceptance window is always chosen tobe above Wl.

For example, if positive signals are being examined the acceptancewindow may be defined between positive Wo 164 and positive Wh 166 andtherefore FIG. 9 illustrates a detection signal 168 produced by anacceptable package on passing the first and second electrodes 2 and 4.The detection signal 168 reaches a peak 170 when the package is disposeduniformly and centrally in the detection zone 10. Any package whichproduces a detection signal having a peak which is less than Wh 166 orgreater than Wo 164 is rejected. A decision is made with respect toevery detection signal which goes above the Wl level 172. The Wl level172 on the return crossing of the detection signal at point 176 as thepackage producing the detection signal leaves the detection zone, isnormally adjusted with respect to the Wl level 172 on the initialcrossing 174 so as to improve the response time enable parts of thedetection circuit 50 to be reset ready for the detection signal producedby the succeeding package in the production line, as describedhereinafter.

The comparator and latch circuit 162 compares the detection signal withthe six threshold levels and outputs the results to six lines 179, 180,181, 182, 183 and 184, respectively. The six lines 179 to 184 correspondto the three positive and negative threshold levels Wo, Wh and Wl,respectively, and if the detection signal passes a threshold level thecorresponding line goes high, otherwise the lines 179 to 184 are low.

The digital word on the lines 179 to 184 is inputted to a decoder 190 ofthe detection circuit 50. The decoder 190 is configured so as todetermine, on the basis of the digital word, whether an accept or rejectcondition has occurred. If the package in the detection zone 10 is to berejected the decoder 190 outputs a positive or negative reject signal,depending on whether a positive or negative detection signal wasanalysed, on line 192 or 194, respectively, to the comparator and latchcircuit 162. The reject signal is stored in a positive or negativereject latch of the comparator and latch circuit 162. The contents ofthe positive and negative reject latches are outputted respectively onlines 196 and 198 to respective inputs 200 and 202 of a NAND gate in aprotection circuit 204 of the detection circuit 50. A rejection output205 from the protection circuit 204 is the output of the NAND gate andis coupled to the triggering input 150 of the reject relay circuit 152,if a digital control switch 208 is closed. Thus the reject relay circuit152 activates the reject relays, discussed previously, whenever thepositive or negative reject latches in the comparator and latch circuit162 are set by the decoder 190.

The detection circuit 50 also includes an amplifier disable circuit 210which monitors the detection signal on the first input 212 and limitsthe low frequency amplifiers 130 and 132 so as to prevent thetransistors of the amplifiers 130 and 132 from operating in thesaturation region, whenever the detection signal exceeds a predeterminedlevel. The amplifier disable circuit 210 also resets the amplifiers 130and 132 whenever a reset signal is received on line 214 from anovershoot latch 216 of the detection circuit 50.

The overshoot latch 216 is used to ensure only one accept of rejectdecision is made with respect to each detection signal received whichcorresponds to a package passed through the detection zone 10. Wheneverthe overshoot protection latch 216 is set a reset signal is outputted online 214 so as to cause the disable circuit 210 to reset the lowfrequency amplifiers 130 and 132. This inhibits the tail of a detectionsignal passed to the comparator and latch circuit 162. Latch outputs 220and 222 of the protection latch 216 also go low and high, respectively,when the protection latch 216 is set so as to reset latches in thecomparator and latch circuit 162 and the decoder 190 and disable thetrigger input 156 to the flow control circuit 154.

The overshoot protection latch 216 includes a set input 224 and a resetinput 226 which are activated when pulled low. The reset input 226 isnormally held low so that the protection latch 216 is normally reset.The reset output 226 goes high whenever a detection signal inputted tothe comparator and latch circuit 216 exceeds the positive or negativelow threshold levels Wl. The reset input 226 is connected to thepositive Wl and negative Wl lines 181 and 184, as shown in FIG. 4.Placing the reset input 226 high enables the overshoot protection latch216 to be set via the set input 224.

The reset input 226 is also connected to a count enable input 228 of theprotection circuit 204. Placing the count enable input 228 high causesthe protection circuit 204 to begin counting in increments from apredetermined value on receipt of the reference signal, from thereference signal amplifier 80, on a clock input 230 of the protectioncircuit 204. If the count ever exceeds a predetermined level, indicatingthe detection signal is not going to return below the positive ornegative low threshold Wl, then a time protection output 232 of theprotection circuit 204 goes low which pulls the set input 224 low andcauses the overshoot protection latch 216 to be set.

The overshoot protection latch 216 is also set when the reject output205 of the protection circuit 204 goes low indicating a reject decisionhas been made, or when an output 240 of the reject relay circuit goeslow indicating that the reject relays have been activated. If an acceptdecision is made on the basis of a positive detection signal or anegative detection signal a respective positive or negative acceptanceoutput 242 or 244 of the decoder 190 goes high so as to activate atransistor 246. Activation of the transistor 246 pulls the set input 224low and thereby sets the overshoot protection latch 216. Thus theovershoot protection latch 216 is set once an accept or reject decisionhas been made.

The reset signal forwarded to the disable circuit 210 on line 214 whenthe protection latch 216 is set is a sampling signal inputted to theovershoot protection latch 216 on line 218 from the protection circuit204. The sampling signal is derived from the reference signal inputtedon the clock input 230 of the protection circuit 204 and has a frequencywhich is a division of the reference signal frequency.

The protection latch 216 is allowed to be reset when the line 250connected between the reset input 226 and the count enable input 228 isno longer held high by a positive or negative Wl line 181 or 184. The Wllines 181 and 184 will be low when the detection signal returns belowthe Wl threshold level. As sufficient information to make an accept orreject decision is normally obtained from the detection signal wellbefore the signal returns below the preset low threshold level Wl, thepreset Wl stored in the comparator and latch circuit 162 is adjusted, orincreased, after the detection signal initially crosses the Wl level, atpoint 174 as illustrated in FIG. 9. This adjustment is performed by thereturn control circuit 160 which receives the detection signal on aninput 260 and uses the incoming detection signal to charge one of twocapacitors, depending on the polarity of the detection signal. Thevoltages across the capacitors are inputted on lines 262 and 264,respectively, to the comparator and latch circuit 162 so as to increasethe positive or negative low threshold level Wl, respectively. Thus,with reference to the example illustrated in FIG. 9, the threshold levelWl is increased so that the return crossing point 176 occurs earlier.This ensures the overshoot protection latch 216 is reset before the nextpackage enters the detection zone 10 and improves the response time ofthe detection circuit. The positive and negative capacitors aredischarged by the reset signal on line 2 and 14 and an inverted versionof the reset signal on a line 266, respectively, when the overshootprotection latch 216 is set.

If, however, the detection signal returns below the low threshold levelWl before an accept or reject decision is made the overshoot protectionlatch 216 is prevented from resetting by a high signal on a zerocrossing output line 270 from the decoder 190. The zero crossing outputline 270 goes high when the detection signal falls between the positiveand negative low threshold levels Wl after having been above the lowlevels. The zero crossing output line 170 remains high until an acceptor reject decision has been made.

When the count enable input 228 goes low after having been high thecount conducted by the protection circuit 244 then begins proceeding indecrements rather than increments. The protection circuit 244 alsobegins counting down when a reject signal is received on an input 272which is connected to the reject latches of the comparator and latchcircuit 162, via the lines 196 and 198. As, in most cases, the detectionsignals received by the detection circuit 50 are symmetrical andperiodic, therefore the count performed by the protection circuit 204should be symmetric. Thus, if desired, the protection circuit 244 can beused to set the overshoot protection latch 216 via the time protectionoutput 232 if the count reaches zero, in order to protect the lowfrequency amplifier 130 and 132 from abnormal fluctuations of thedetection signal.

The inputs to the hold control circuit 158, discussed previously, are alow threshold comparator output 280 from the comparator and latchcircuit 162, a zero crossing output 282 from the decoder 190 and thelines 196 and 198 which are connected to the positive and negativereject latches of the comparator and latch circuit 162. The hold controlcircuit 158 can also be used to provide a reject inhibit signal on aninhibit line 284 which activates the transistor 246 so as to set theovershoot protection latch 216 when the reset input 226 is high.

A prototype of the apparatus of the invention has been tested and foundto be capable of very accurate monitoring of the number articles in apackage. For instance it has been tested with packages or gauze swabsfor surgical use. The swabs are 7.5×7.5 cm and typically five or ten areprovided in a package. It is very important to know whether througherror an additional swab has been included in the package. If anadditional swab is present there is a possibility that it may be leftinside the body of a patient being operated upon. It is the practice oftheatre staff to count the number of used swabs at the end of anoperation to check that all have been removed from the patient. It willbe appreciated that, if the wrong number of swabs is present in a pack,the usual checking procedure adopted may lead to error.

The apparatus of the invention can be used to scan for defects orinclusions in products, for instance air bubbles in confectionary orforeign bodies in solids for instance pieces of stone in bars of soap.The resolution monitoring in a direction transverse to the direction ofmovement of the articles is dependent on the number of secondelectrodes.

It will be appreciated that the second electrode 4 is narrow so that theinspection zone 10 is likewise narrow. The products can be introducedinto the zone at a place where the products should be uniform. Thisenables accurate counting or monitoring to be carried out.

The principles of the invention can be applied to flow meters for solidsor liquids passing through a conduit. The output signals from theelectrode or electrodes can be integrated over time to give anindication of flow.

Many modifications will be apparent to those skilled in the art withoutdeparting from the spirit and scope of the invention.

We claim:
 1. Apparatus for detecting the amount or presence of materialin a detection zone (10), said apparatus comprising:a first electrode(2); input signal means (14, 52, 54) for supplying an electric inputsignal to the first electrode; a second electrode (4), said detectionzone (10) being located between said first and second electrode (2 and4); a third electrode (6) which substantially surrounds the secondelectrode (4) and is substantially larger than the second electrode (4);and receiving means (16, 58) for detecting signals on the secondelectrode (4), the signal on the second electrode (4) being responsiveto material in the zone (10), said receiving means including a high gainamplifier (16) coupled to the second and third electrodes (4, 6) formaintaining said second and third electrodes at substantially the sameelectrical potential thereby to establish a substantially linearelectric field between said first electrode (2) and said secondelectrode.
 2. Apparatus as claimed in claim 1, wherein the second andthird electrode and said high gain amplifier operate, in combination,for limiting fringing fields to occur only between the first and thirdelectrode (2 and 6) at their outer edges.
 3. Apparatus as claimed inclaim 1, wherein said second electrode (4) is narrow so that a narrowslice of material in the zone (10) is detected.
 4. Apparatus as claimedin claim 3, wherein said apparatus is configured to operate on aproduction line, wherein packaged articles pass through the detectionzone (10), said first electrode (2) comprises a plate over which thearticles pass and the second and third electrodes (4 and 6) are locatedabove the first electrode (2).
 5. Apparatus as claimed in claim 1,wherein said amplifier (16) comprises an operational amplifier (16, 62)with one input (15, 60) connected to said second electrode (4) andanother input (17) connected to said third electrode (6) and ground,said amplifier (16, 62) having a capacitor (20) in a feedback loopbetween said one input (15, 62) and the output (22) of the amplifier(16, 62).
 6. Apparatus as claimed in claim 5, further comprising means(144, 162, 190) for determining whether an acceptable amount of materialis present in said zone (10) using the signal outputted by the amplifier(16, 62).
 7. Apparatus as claimed in claim 6, wherein said input signalis a periodic signal having a predetermined frequency.
 8. Apparatus asclaimed in claim 7, wherein said input signal is a sinusoidal signal. 9.Apparatus as claimed in claim 7, further comprising means (74) forsampling and filtering the signal outputted by the amplifier (16, 52) soas to produce a detection signal having an amplitude which isrepresentative of the amount of material in said detection zone (10) andis supplied to said detection means (144, 162, 190).
 10. Apparatus asclaimed in claim 9, wherein said detection means (144, 162, 190)includes a level detector which triggers means (152) for rejecting thematerial in said zone if said detection signal exceeds a predeterminedthreshold level.
 11. Apparatus as claimed in claim 9, wherein saiddetection means (144, 162, 190) includes a decoder means (190) fordetermining if said detection signal lies within an acceptance window,and if said detection signal lies outside said acception window, saiddecoder means (190) issues a signal which cause said means (152) forrejecting the material in said zone (10) to reject said material. 12.Apparatus as claimed in claim 11, further comprising means (216, 204)for disabling the detection means (144, 162, 190) once a decision hasbeen to accept or reject material in said zone (10) to ensure thedetection signal is not further analysed to reverse the decision.
 13. Adetection device for use in detecting the amount or presence of materialin a detection zone (10), said device comprising:a first electrode (2);a second electrode (4) having a narrow dimension relative to said firstelectrode and substantially parallel with respect to and opposite saidfirst electrode (2); and a third electrode (6) which substantiallysurrounds, is substantially electrically isolated with respect to and issubstantially larger than said second electrode (4); said detection zone(10) being located between said first electrode (2) and said secondelectrode (4), said receiving means including a high gain amplifier (16)coupled to the second and third electrodes (4, 6) for maintaining saidsecond and third electrodes at substantially the same electricalpotential thereby to establish a substantially linear electric fieldbetween said first electrode (2) and said second electrode. 14.Apparatus for detecting the amount or presence of material in adetection zone, said apparatus comprising:a first electrode; inputsignal means for supplying an electric input signal to the firstelectrode; a second electrode, said detection zone being located betweensaid first and second electrodes; a third electrode closely spaced fromthe second electrode and substantially larger than the second electrode;and receiving means for detecting signals on the second electrode, thesignal on the second electrode being responsive to material in the zone,said receiving means including a high gain amplifier (16) coupled to thesecond and third electrodes (4, 6) for maintaining said second and thirdelectrodes at substantially the same electrical potential thereby toestablish a substantially linear electric field between the firstelectrode (2) and the second electrode.
 15. Apparatus for detecting theamount or presence of material in a detection zone, said apparatuscomprising:a first plate electrode; input signal means for supplying anelectric input signal to the first electrode; a second electrode, saiddetection zone being located between said first and second electrodes; athird plate electrode parallel to and spaced from the first electrode,said third electrode being closely spaced from the second electrode andsubstantially larger than the second electrode; and receiving means fordetecting signals on the second electrode, the signal on the secondelectrode being responsive to material in the zone, said receiving meansincluding a high gain amplifier (16) coupled to the second and thirdelectrodes (4, 6) for maintaining said second and third electrodes atsubstantially the same electrical potential thereby to establish asubstantially linear electric field between the first electrode (2) andthe second electrode.
 16. Apparatus for detecting the amount or presenceof material passing through a detection zone in a first direction, saidapparatus comprising:first electrode means; input signal means forsupplying an electric input signal to the first electrode means; secondelectrode means, said detection zone being located between said firstand second electrode means the second electrode means being narrowrelative to said first electrode means and extending parallel to saidfirst direction; third electrode means closely adjacent to the secondelectrode means and being substantially larger than the second electrodemeans; and receiving means for detecting signals on the second electrodemeans, the signal on the second electrode means being responsive tomaterial in the zone, said receiving means including a high gainamplifier (16) coupled to the second and third electrode means (4, 6)for maintaining said second and third electrode means at substantiallythe same electrical potential thereby to establish a substantiallylinear electric field between the first electrode means (2) and thesecond electrode means.